Diastereo‐ and Regioisomeric Bicyclic Thiohydantions from Chiral 1,3‐Thiazolidine‐2,4‐dicarboxylic acids

Bicyclic thiohydantoins were synthesized in a stereoselective manner by reacting (2R)/(2S)-diastereoisomer mixtures of 1,3-thiazolidine-2,4-dicarboxylic acids or their dimethyl diesters with PhNCS. 5,5-Dimethyl-1,3-thiazolidine-2,4-dicarboxylic acid with PhNCS led to a cyclization involving the CO group at the C(2) center of the thiazolidine ring, while the acid's dimethyl diester gave cyclization involving the CO group at C(4). In contrast, reactions involving unsubstituted 1,3-thiazolidine-2,4-dicarboxylic acid or its dimethyl diester led to thiohydantoins in which the ring closure had taken place only with the COO group at C(4). Independently of the direction of the ring closure, all reactions produce exclusively products with the (R)-configuration at C(2). The configurational assignments were based on 1H- and 13C-NMR studies, and confirmed by X-ray crystallographic analyses.